1. Field of the Invention
The present invention relates generally to the control of the delivery of power in a variety of applications, and more particularly to methods and systems to deliver and control pulsed power as may be applicable to plasma thin film processing and other applications.
2. Brief Description of the Prior Art
Thin film layer systems are commonly deposited on glass or polymer substrates with a large area coater. These layer systems are often an implementation of an optical filter. These filters use a combination of interference effects and the transmission and reflection spectra of the discrete metal and dielectric layers to arrive at their composite spectral transmission and absorption characteristics. The metallic layers, commonly titanium, silver, nichrome, aluminum or stainless steel, are typically sputtered with a magnetron operating in the metallic mode in an argon gas environment and powered by a DC supply as described in the book by R. J. Hill, S. J. Nadel, xe2x80x9cCoated glass applications and marketsxe2x80x9d, BOC (1999), p.55-86
A process known as reactive sputtering, where a metal or semiconductor is sputtered in the presence of a reactive gas, typically oxygen or nitrogen, deposits the dielectric layers. The gas then combines with the conductive sputtered material to form a dielectric. This dielectric tends to be deposited on the sputtering target and anode as well as the work piece. The result is an insulating coating on both the target and the anode, which will eventually degrade and perhaps even shut down the process when it is powered by a DC supply. This degradation is primarily due to an effect referred to as a xe2x80x9cdisappearing anodexe2x80x9d. The anode will disappear because it is eventually coated with an insulator, the same reactively formed dielectric compound deposited on the work piece.
One solution to this problem is to use a dual magnetron sputtering arrangement, as shown in FIG. 1. In this approach the pair of magnetrons A and B is driven by an AC supply that is electrically isolated from the plasma chamber 4 creating plasma 6. Therefore, they alternate roles between cathode and anode. So, after a brief time acting as an anode, and receiving a tiny deposition of dielectric, the magnetron will act as a cathode, sputtering conductive material as well as the little bit of dielectric that was deposited during the time it acted as an anode. As a result, a clean anode is always available to complete the current path.
Some industrial applications require delivery of power in a pulsed format where the power delivered in opposite polarities must be regulated independently. The pulsed dual magnetron sputtering arrangement used for depositing thin film coatings is such an example. The two magnetrons labeled xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d are situated in the plasma chamber, in a low-pressure gas environment. The power supply is connected to the magnetrons through gas-tight insulating feed-throughs. Loads of this type can have different voltages for each polarity. The voltages also vary dynamically with time and non-linearly with current. One example of a current versus voltage (I-V) curve for a dual magnetron arrangement is shown in FIG. 2. The current may vary non-linearly with voltage and I-V characteristics for positive and negative voltages may not be symmetrical. An example of the pulsed voltage and current waveforms as a function of time is shown in FIG. 3. The power supply can be set to have different amplitudes and pulse widths for positive and negative polarities, which can provide the capability of regulating the power delivered to each magnetron independently. The positive current can be smaller in proportion to the positive voltage than the negative current in proportion to the negative voltage. This is because the I-V characteristics for positive and negative voltages may not be symmetrical, as shown in FIG. 3. In practical applications these loads can vary in steady state operating voltage, and therefore large signal impedance, depending on physical configuration, magnetic field strength, process gas composition and pressure, target composition, and steady state current.
This technique was reported in G. Este, W. D. Westwood, J. Vac. Sci. Technology, A 6 (3), (May/June 1988), p. 1845-1848. Now, commonly referred to as dual magnetron sputtering (DMS) or dual cathode sputtering. This configuration is widely used in the deposition of low-emissivity (xe2x80x9clow-exe2x80x9d) coatings on architectural glass in large in-line coaters. Other significant applications include mirrors, flat panel displays, and anti-reflection (AR) coated glass. DMS is also employed in roll coaters for depositing coatings on plastic films for stick-on glare reduction filters as well as oxygen barriers for plastic food packaging films. An additional benefit of dual magnetron sputtering is the denser and smoother films it produces, due to a much higher flux of energetic ions at the substrate. Silver layers deposited on these smoother films have lower sheet resistance, and therefore lower emissivity, with the same optical transmission characteristics as taught in H. Schilling, et al., xe2x80x98New layer system for architectural glass based on dual twin-magnetron sputtered TiO2xe2x80x99, 41st Annual Technical Conf. Proc., SVC (1998), p. 165-173.
Considerable effort has been expended to develop power supplies whose topologies are optimized for driving these dual magnetron systems. Both square-wave pulsed supplies and sinusoidal (or, resonant) power supplies have been used. High power sinusoidal AC supplies are commercially available. Descriptions of these power supplies have appeared in the technical literature and have been presented at conferences within the past few years as disclosed in T. Rettich, P. Wiedemuth, Journal of Non-Crystalline Solids 218 (1997) p. 50-53; T. Rettich, P. Wiedemuth, xe2x80x98New application of medium frequency sputtering for large area coatingxe2x80x99, 41st Annual Technical Conf. Proc., SVC (1998), p. 182-186; and G. Wallace, Thin Solid Films 351 (1999) p. 21-26. The AC power supplies on the market today control and measure the total power delivered to the two magnetrons. Measurement and control of the power, current and voltage for individual magnetrons is not yet available in sinusoidal AC supplies.
Pulsed supplies inherently offer more flexibility in control of the process. They provide the capability of independently regulating the power delivered to each magnetron. This has some advantages for existing processes, and enables the implementation of new processes. First, independent regulation of power to each magnetron can force each magnetron to receive the same power. Consequently, the racetracks erode at the same rate. When a resonant supply is used, an impedance difference between the two magnetrons can result in faster erosion of one target, which unnecessarily reduces the time between preventative maintenance cycles. Second, it is possible to intentionally operate the two magnetrons at different powers. If one target develops a tendency to arc, its power can be reduced to the point where it arcs at an acceptable rate, and, sometimes, the power to the other magnetron can be increased to compensate and maintain the same deposition rate from the pair. Third, independent regulation enables the creation of controlled mixtures of materials in the film when dissimilar materials are used for the magnetron targets. This would allow the creation of films with customized or graded indexes of refraction. For example, SiO2 can be deposited with a refractive index of about 1.5 and TiO2 can be deposited with a refractive index of about 2.4. If a dual magnetron sputtering arrangement is configured with one Si target and one Ti target, the ratio of Ti to Si can be controlled by controlling the power to each of the magnetrons. Therefore, in principle, it is possible to xe2x80x9cdialxe2x80x9d the refractive index anywhere between 1.5 and 2.4.
Pulsed power supplies used in dual magnetron sputtering need to be rated at 120 to 200 kW with the ability to regulate on voltage, current, or power are disclosed in P. Greene, R. Dannenberg, xe2x80x98Modelling of production scale reactive depositionxe2x80x99, 42nd Annual Technical Conf. Proc., SVC (1999) p. 23-28; and U. Heister, et al., xe2x80x98Recent developments on optical coatings sputtered by dual magnetron using a process regulation systemxe2x80x99, 42nd Annual Technical Conf. Proc., SVC (1999), p. 34-38. These supplies should be able to deliver full power over a 2 to 1 voltage range and must have sophisticated arc management capability offering arc prevention and recovery and very low arc energy. They should also offer variable frequency. In general, a process should be run at the lowest frequency consistent with an acceptable arc rate in order to achieve the highest deposition rate. Some processes require periodic cleaning cycles, and, in this case, the ability to run at higher frequencies for cleaning is desirable.
A square wave voltage source supply has been disclosed in U.S. Pat. No. 5,303,139 issued in April 1994 to G. Mark, entitled xe2x80x9cLow frequency, pulsed, bipolar power supply for a plasma chamberxe2x80x9d. Pulsed voltage source supplies typically demonstrate slow current rise and high peak currents into an arc. Consequently, they have had limited commercial acceptance for the dual magnetron sputtering application. Further work has focused on the development of pulsed current source supplies as disclosed in U.S. Pat. No. 5,777,863 issued July, 1998 to D. Kovalevskii and M. Kishinevsky, entitled xe2x80x9cLow-frequency modulated current mode power supply for magnetron sputtering cathodesxe2x80x9d; U.S. Pat. No. 5,917,286 issued June, 1999 to R. A. Scholl and D. J. Christie, entitled xe2x80x9cPulsed direct current power supply configurations for generating plasmasxe2x80x9d; and U.S. Pat. No. 6,005,218 issued December 1999 to H. Walde, et al. entitled xe2x80x9cProcess and circuit for the bipolar pulse-shaped feeding of energy into low pressure plasmas.
One problem with asymmetric bipolar pulsed supplies is ground insulation. Those skilled in the art know that a simple output transformer similar to those used in AC power supplies may saturate when voltage waveform applied to the primary winding is not balanced, meaning that average input voltage is not zero. In the above-mentioned patent of Kovalevskii et al, using a low frequency isolating transformer at the input section of the power supply solved this problem. First, a low frequency transformer tends to be very large and heavy at high power requirements. Second, absence of an output transformer reduces the load voltage range of the power supply since the load cannot be matched by changing transformer taps.
It is an object of this invention to provide a way to use an output transformer with a pulsed power supply while preventing saturation.
It is another object of this invention to provide an output transformer that would allow for broader load range and more efficient use of power devices in the pulsed power supply.
It is yet another object of this invention to provide a power supply that has a smaller size, footprint, and weight due to the elimination of the low frequency input transformer.
There is provided by this invention a novel pulsed DC power supply for dual magnetron sputtering applications that utilizes a transformer that provides the functions of isolation and voltage level transformation. This provides isolation with the possibility of transforming the output voltage of the unit to a different level. The power supply output current and the voltage-current (V-I) characteristics of the process load determine the output voltage. A Hall effect sensor is used to directly sense the flux in the magnetic circuit. A control circuit uses this signal to set the duty cycle of a power supply to achieve the condition where the average voltage on the primary of the transformer is zero, a necessary condition to prevent the transformer from saturating. Additional circuitry prevents saturation during a single pulse.